In many optical systems, a diffraction grating is used to spread light into a spectrum. The spectrum from the diffraction grating can be projected onto a screen or an array of detectors to allow the spectrum to be viewed or analyzed. The spectrum is spatially distributed along a spectral axis on the screen or the array of detectors.
Because of the periodic nature of the diffraction grating, the spectrum from the grating is actually composed of many replicated spectra, each individual spectrum being of a different integral order. In the general case, the individual spectra spatially overlap each other on the screen or array of detectors along the spectral axis. For example, the blue line in the second-order spectrum may be projected within the red portion of the first-order spectrum. In many applications, this overlap is undesirable. It is usually desirable to be able to analyze a single spectrum by itself without interference from spectra of different orders.
Techniques have been devised for eliminating overlap among the spectra in order to facilitate analysis. One technique uses band pass filters to eliminate the overlap. A second technique is known as cross dispersion. Cross dispersion involves dispersing the spectrum in the direction orthogonal to the spectral axis. The different order spectra are spatially displaced above or below each other. Thus, the spectrum of interest is left to be analyzed without interference from the other spectra.
Several drawbacks attend this approach. First, in many applications, both dimensions in the screen or the array of detectors are used to view or analyze the spectrum. Analysis is performed along both the spectral axis and a spatial axis orthogonal to the spectral axis. For instance, in many imaging spectrometers, the light from an extended object is imaged onto an entrance slit of the spectrometer oriented along a spatial axis. The light is then spectrally dispersed by the spectrometer in a direction along the spectral axis perpendicular to the slit or spatial axis. Thus, the direction perpendicular to the slit (spectral axis) contains the spectral information while the direction along the slit (spatial axis) contains the spatial information for a given strip of the target object. Since the direction orthogonal to the spectrum is used for spatial information, the higher-order spectra cannot be displaced in this direction.
Another drawback of cross dispersion involves the phenomenon known as "smile." If the higher-order spectra are displaced orthogonally away from the lowest-order spectrum by a refractive element such as a prism, then the short wavelength light is dispersed by the refractive element more than long wavelength light. This property has detrimental effects on the technique. The spectra tend to bend into a "smile." Thus, the desired one-dimensional mapping of the spectra for each point of light is distorted into a two-dimensional curved mapping.